Many types of wireless device are known in which data is transmitted and received over a radio-frequency (RF) communications link. For example, WI-FI (Registered Trademark) or frequency hopping spread spectrum communication systems, such as Bluetooth (Registered Trademark), are widely used for establishing wireless computer networks. It has also been described previously how a Bluetooth communications module can be activated by an associated low power receiver in the form of a radio-frequency identification (RFID) tag. For example, US2007/0015465 describes an electronic RFID tag that can be used to activate a Bluetooth module. Similar types of arrangement have also been proposed previously in US2003/107475 and WO03/065289.
In the field of metrology, it is also known to transmit measurement data from a battery operated measurement probe to an associated interface over a wireless RF link. The provision of such a wireless link is especially useful where the hardwiring of a measurement probe is inconvenient or simply not possible. For example, the use of a hardwired solution is simply impractical for measurement probes that are periodically loaded into the spindle of a standard machine tool using automated tool change apparatus or the like.
The wireless systems provided in the majority of commercially available measurement probes have, to date, tended to be based on fixed frequency transmission systems. However, bespoke spread spectrum communication systems have also been developed for such applications. An example of a spread spectrum radio system designed for use with a position measurement probe is outlined in WO2004/057552 (Renishaw plc). The system of WO2004/057552 uses a transmission protocol that is designed to provide reliable transmission of measurement probe data whilst ensuring clocks provided at the interface and measurement probe remain synchronised to ensure measurement accuracy.
Providing wireless measurement probes overcomes the problems associated with hardwiring, but introduces the requirement to periodically replace or recharge the batteries of the measurement probe. Taking a machine tool off-line to change the batteries of a measurement probe can be disruptive to a production process and also requires the intervention of a machine operator. WO2004/057552 describes how the spread spectrum radio module of the battery operated measurement probe can operate in either a normal mode or a power saving mode in which the rate of transmission of the regular signals is reduced. Although the battery power consumed by such a system when placed in standby mode is low, it is not negligible and battery life can therefore still be an issue.